The present invention relates to a lightweight, flexible WAFFLELINE.RTM. structure (WAFFLELINE is a registered trademark of Harris Corporation) and, more particularly, to a WAFFLELINE.RTM. structure (WAFFLELINE is a registered trademark of Harris Corporation) having particular use as an RF interconnect structure which permits the inclusion of highly integrated RF/microwave functions into a structure having flat or curved surfaces such as may be found, for example, in the smart skin of aircraft fuselages. The present invention is also directed to the method of making such a lightweight, flexible WAFFLELINE structure using a silicone casting as a negative matrix, to reproduce the WAFFLELINE structure either with epoxy resin castings or electroformed nickel with 0.degree. draft on the WAFFLELINE channel walls and with weight savings of 250% or more over conventional WAFFLELINE structures which have been machined.
Typically, a WAFFLELINE structure is fabricated from a machined metallic structure such as aluminum. As shown in U.S. Pat. No. 4,695,810, WAFFLELINE grid structures have been produced by machining or etching an aluminum plate. For example, a multi-blade circular saw mounted on a single arbor in an automatic feed milling machine cuts grooves in the aluminum plate to form parallel, spaced-apart channels through sequential cuts. After the grooves are cut in one direction, the plate is then rotated 90.degree. for cutting other parallel, spaced-apart channels so as to define a matrix of substantially rectangular plate material therebetween.
Another technique which has been used is investment casting of the metal plate because it requires very little machining clean-up. All design features, including the shape, configuration and component mounting pockets, as well as the WAFFLELINE grid structure, can be formed simultaneously. Similarly, injection molding has been used to produce the grid structure.
The problem with the foregoing techniques of fabricating a WAFFLELINE structure is that they are relatively expensive processes. In addition, due to tolerance variations, it is not possible to produce identical WAFFLELINE structures when machine cutting is used. The conventional WAFFLELINE structure made from metal plate is also relatively thick and consequently undesirably heavy. Although the metal plate provides good heat transfer characteristics, its thickness and weight make it undesirable in many aerospace applications where space and weight are critical factors. This has become a significant disadvantage as WAFFLELINE structure details have become much smaller for high frequency applications and as it has become even more critical to maintain the geometries of the mesas and channels. Another disadvantage of conventional WAFFLELINE structures is that, because they are made of metal, they are inflexible. Thus, they cannot be conveniently used with different types of conformal surfaces, such as smart skin on aircraft fuselages.
It is an object of the present invention to overcome the problems and disadvantages associated with conventional WAFFLELINE structures and to provide a lightweight, flexible WAFFLELINE interconnect structure and method of producing same which allows the inclusion of highly integrated RF/microwave functions into a structure which is conformal with respect to curved surfaces and which can be fabricated from a cast composite structure or a nickel electroform structure.
In particular, it is an object of the present invention to provide a WAFFLELINE structure in which the weight is reduced on the order of two to three times the weight of machined structures with a relatively simple and inexpensive casting technique which uses a negative WAFFLELINE structure image made of a casting mixture such as silicone rubber, permits a 0.degree. draft on WAFFLELINE structure channel walls and greatly simplifies the process of ejecting the finished WAFFLELINE structure from the negative WAFFLELINE structure image, yet provides outstanding resolution of the waffleline details.
The foregoing objects have been achieved in accordance with the present invention by producing an extremely lightweight resin casting or an electroformed nickel for the WAFFLELINE structure and utilizing a silicone negative mold in a casting process to form the very thin WAFFLELINE structure which also results unexpectedly in a flexible WAFFLELINE structure which can be used on non-flat surfaces. A first step in the process involves the machining of a tooling plate in the form of a relatively thick and rigid metal positive WAFFLELINE structure having the matrix details. A second step involves using the metal positive image onto which silicone is cast to form a negative mold with a silicone rubber. A composite epoxy resin can then be poured into the silicone mold in a mold assembly to form the WAFFLELINE structure a thin, flexible waffleline interconnect structure. Alternatively, instead of the second casting step using epoxy resin, nickel can be electroformed over the silicone mold in a known manner to produce the thin, flexible structure.
The present invention also contemplates that the epoxy resin can either be provided with a conductive filler which, of course, will add some weight or can be metallized so that it will conduct RF energy.
The advantages of the foregoing method and product are that the WAFFLELINE structure can be manufactured with an overall height less than 0.05 inches and a 0.degree. draft angle. Although the WAFFLELINE structure can be made very thin and lightweight, it is also releasable from the silicone mold without a complicated ejection system. The 0.degree. draft angle insures a proper wire restraint in the channels which also yields good RF transmission properties. The thin composite casting or nickel electroform represents a significant weight savings compared to the conventional metallic WAFFLELINE structure with the added advantage of flexibility for conformal surfaces.
A composite epoxy resin can be, for example, a two part resin such as EA 956 with Carbospheres for blackening the surface of the WAFFLELINE structure. This particular composite epoxy cures at room temperature which is also another advantage. It will be readily appreciated, however, that many other different types of composite epoxy resins can be used depending upon the particular application and the application requirements.
When a nickel electroform version is used, the electroform itself will be one to three mils thick. The nickel will be provided with a thin silver layer for conducting energy. Where epoxy is used to produce the WAFFLELINE structure for carrying wires, the structure can be made at least 50% lighter than machined aluminum and, in some instances, can be up to 300% lighter than a conventional machined aluminum WAFFLELINE. In addition, WAFFLELINE structures can now be matched with composite epoxy structures to which they are attached to permit bonding therebetween.